self._manager is None when proxy gets passed between processes and causes AttributeError

Reproduce the failure with the minimal test.py example, then inspect Lib/multiprocessing/managers.py around BaseProxy._callmethod and the linked PR gh-99598. Done means a proxy passed to another process can call make_child without the AttributeError and the regression is covered by a test.

Bug report

When a proxy derived from multiprocessing.managers.BaseProxy is passed between processes, the self._manager attribute becomes None. self._manager is however accessed to retrieve its _registry attribute, which contain the method_to_typeid map recorded into the manager viaBaseManager.register(), causing an AttributeError. The solution is to decouple the role of the manager from the role of the registry and propagate the registry separately. I'll submit a pull request to fix this.

Minimal failing example

test.py

from multiprocessing import Process
from multiprocessing.managers import BaseProxy, BaseManager


class Parent(object):
    def __init__(self, *args, **kwargs):
        pass

    def make_child(self):
        return Parent()


class MyManager(BaseManager):
    pass


def child_generator(parent):
    parent.make_child()


if __name__ == "__main__":
    MyManager.register('Parent', callable=Parent,
                       method_to_typeid={'make_child': 'Parent'})

    with MyManager() as manager:
        parent = manager.Parent()
        process = Process(target=child_generator, args=(parent, ))
        process.start()
        process.join()

Output

❯ python3 test.py
Process Process-2:
Traceback (most recent call last):
  File "/usr/local/Cellar/python@3.10/3.10.8/Frameworks/Python.framework/Versions/3.10/lib/python3.10/multiprocessing/process.py", line 314, in _bootstrap
    self.run()
  File "/usr/local/Cellar/python@3.10/3.10.8/Frameworks/Python.framework/Versions/3.10/lib/python3.10/multiprocessing/process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "test.py", line 18, in child_generator
    parent.make_child()
  File "<string>", line 2, in make_child
  File "/usr/local/Cellar/python@3.10/3.10.8/Frameworks/Python.framework/Versions/3.10/lib/python3.10/multiprocessing/managers.py", line 824, in _callmethod
    proxytype = self._manager._registry[token.typeid][-1]
AttributeError: 'NoneType' object has no attribute '_registry'

Environment

• CPython versions tested on: 3.6, 3.7, 3.8, 3.9, 3.10
• Operating system and architecture: macOS Monterey 12.6

Linked PRs

• gh-99598